RFID transponders are used in numerous applications such as identification or passive keyless entry systems. A field of application is a road vehicle keyless entry system. Such an entry system uses RF-signals in the frequency range of about 120 kHz to 140 kHz. Typically, a base station transmitter that is located in the car sends a short sequence of symbols as digitally modulated RF-signals. A known method of modulation is amplitude shift keying. A mobile transponder that is typically a part of a car key receives these RF-signals and if the detected pattern matches the expected value, it answers using UHF frequencies that are typically in the band of 315 MHz in the USA and 433 MHz in Europe. After the initial sequence, more information is exchanged between the base station and the transponder according to an authentication protocol using either the low frequency RF band or the UHF band. When the transponder has successfully authenticated itself to the base station, the car doors may be unlocked and engine start may be allowed. A typical transponder system for such a remote keyless entry system is known from U.S. Pat. No. 6,323,566 B1, filed on Oct. 10, 1996.
The most valuable and limited resource in any RFID transponder is power or energy that may be provided by a battery. For reasonable battery lifetime, the transponder must consume very little power when listening to an initial sequence. When the initial sequence is detected, the transponder activates the UHF transmitter and the cryptographic protocol circuitry. In this activated state, the RFID transponder consumes much more power. The starting up of the transponder is also called a wake up, the initial sequence is typically referred to as a wake up pattern.
FIG. 1 shows a schematic layout of an RFID transponder 1 comprising at least one receiver channel 6 having an analog front end 2 and a digital wake up pattern detector 4 for detecting a wake up pattern. Typically, three receiver channels 6, one for every direction in space, are build in a 3D receiver for a mobile transponder. The front end 2 receives an RF-signal RF from a suitable antenna 12. The automatic gain control AGC amplifies the signal RF that is subsequently demodulated by a demodulator 8. A bit detector 10 that is itself not a part of the analog front end 2 but interacts with it, provides a data signal D to the wake up pattern detector 4. Upon successful detection of a wake up pattern in the data signal D, the wake up pattern detector 4 sends a wake signal W to the further parts 12 of the RFID transponder 1. Inter alia, a subsequent bit detector is started and evaluates the received ASK information in a demodulation mode and according to the used symbol coding.
An RFID transponder with improved wake up pattern detection is known from DE 10 2008 060 082 filed on Dec. 2, 2008.